AQA A-level Geography · 3.1.1

Drainage basins, water balance and hydrographs

Exam-style questions with answers for 3.1.1 Drainage basins, water balance and hydrographs in AQA A-level Geography (spec 7037). Practise with flashcards, a topic quiz, a random quiz across the course, 5 crosswords, or read every question and answer below.

All questions and answers

Question 1 · Spec 3.1.1.2 · Drainage basin system
Define a drainage basin. [2 marks]
Answer:
The area of land drained by a river and its tributaries (1); bounded by a watershed, the high ground that separates it from neighbouring basins (1).
Question 2 · Spec 3.1.1.2 · Drainage basin system
State the main input and two main outputs of a drainage basin. [3 marks]
Answer:
Input: precipitation (1); outputs: evapotranspiration (1); runoff (discharge) to the sea or a lake (1).
Question 3 · Spec 3.1.1.2 · Drainage basin stores
Name four stores of water in a drainage basin. [4 marks]
Answer:
Any four from: interception (1); surface storage (1); soil water (1); groundwater (1); channel storage (1); vegetation storage (1).
Question 4 · Spec 3.1.1.2 · Drainage basin stores
Define interception. [2 marks]
Answer:
Precipitation caught and held temporarily by vegetation (1); before it evaporates or drips to the ground as throughfall or flows down stems (1).
Question 5 · Spec 3.1.1.2 · Drainage basin stores
Define surface storage. [1 mark]
Answer:
Water held on the ground surface in puddles, ponds and lakes.
Question 6 · Spec 3.1.1.2 · Drainage basin stores
Define channel storage. [1 mark]
Answer:
Water held within river channels.
Question 7 · Spec 3.1.1.2 · Drainage basin flows
Name four flows (transfers) of water in a drainage basin. [4 marks]
Answer:
Any four from: stemflow (1); infiltration (1); percolation (1); throughflow (1); overland flow (1); groundwater flow (1); channel flow (1).
Question 8 · Spec 3.1.1.2 · Drainage basin flows
Define stemflow. [1 mark]
Answer:
Water flowing down the stems and trunks of plants to the ground.
Question 9 · Spec 3.1.1.2 · Drainage basin flows
Define infiltration. [1 mark]
Answer:
The movement of water from the ground surface into the soil.
Question 10 · Spec 3.1.1.2 · Drainage basin flows
Explain three factors that affect the infiltration rate. [3 marks]
Answer:
Any three from: soil type, as sandy soils with large pore spaces allow faster infiltration than clay (1); antecedent moisture, as saturated soil cannot absorb more water (1); vegetation cover, as roots open channels in the soil (1); compaction or impermeable surfaces reduce infiltration (1); rainfall intensity, as heavy rain can exceed infiltration capacity (1).
Question 11 · Spec 3.1.1.2 · Drainage basin flows
Define percolation. [1 mark]
Answer:
The downward movement of water from the soil into underlying permeable rock.
Question 12 · Spec 3.1.1.2 · Drainage basin flows
Define throughflow. [1 mark]
Answer:
The lateral movement of water through the soil towards a river channel.
Question 13 · Spec 3.1.1.2 · Drainage basin flows
Distinguish between infiltration-excess and saturation-excess overland flow. [2 marks]
Answer:
Infiltration-excess occurs when rainfall intensity is greater than the infiltration capacity of the soil (1); saturation-excess occurs when the soil is already full of water so extra rain runs over the surface (1).
Question 14 · Spec 3.1.1.2 · Drainage basin flows
Define groundwater flow (baseflow). [2 marks]
Answer:
The slow movement of water through saturated permeable rock (1); which feeds rivers and sustains their flow in dry periods (1).
Question 15 · Spec 3.1.1.2 · Drainage basin flows
Explain why overland flow reaches a river faster than groundwater flow. [2 marks]
Answer:
Overland flow travels across the surface with little resistance, often over short distances (1); groundwater flow is slowed by friction as it moves through tiny gaps in rock and soil (1).
Question 16 · Spec 3.1.1.2 · Water balance
Define the drainage basin water balance. [2 marks]
Answer:
The relationship between inputs and outputs of water in a drainage basin (1); taking account of changes in the amount of water stored (1).
Question 17 · Spec 3.1.1.2 · Water balance
State the water balance equation and define its terms. [2 marks]
Answer:
P = Q + E ± ΔS (1); where P is precipitation, Q is runoff (discharge), E is evapotranspiration and ΔS is the change in storage (1).
Question 18 · Spec 3.1.1.2 · Water balance
A drainage basin receives 1,000 mm of precipitation in a year. Evapotranspiration is 600 mm and runoff is 380 mm. Calculate the change in storage. [2 marks]
Answer:
ΔS = P − (Q + E) = 1,000 − (380 + 600) (1); = +20 mm, so storage has increased (1).
Question 19 · Spec 3.1.1.2 · Water balance
A basin has 750 mm of precipitation and 250 mm of runoff in a year, and no change in storage. Calculate evapotranspiration. [2 marks]
Answer:
E = P − Q (1); = 500 mm (1).
Question 20 · Spec 3.1.1.2 · Water balance
Define field capacity. [1 mark]
Answer:
The maximum amount of water a soil can hold against gravity, after excess water has drained away.
Question 21 · Spec 3.1.1.2 · Water balance
Describe the four phases of the soil moisture budget in the UK. [4 marks]
Answer:
Soil water surplus in winter, when precipitation exceeds potential evapotranspiration and the soil is at field capacity (1); soil moisture utilisation in spring and summer, as plants use stored water (1); soil water deficit in late summer, when the soil is dry and plants are stressed (1); soil moisture recharge in autumn, as rainfall refills the soil (1).
Question 22 · Spec 3.1.1.2 · Water balance
Explain why a soil water surplus leads to more runoff. [2 marks]
Answer:
The soil is saturated at field capacity so cannot store more water (1); extra water flows over the surface or through the soil to rivers (1).
Question 23 · Spec 3.1.1.2 · Flood hydrograph
What is a flood (storm) hydrograph? [1 mark]
Answer:
A graph showing how river discharge changes over time at a point on a river, in response to a rainfall event.
Question 24 · Spec 3.1.1.2 · Flood hydrograph
Describe the rising limb, peak discharge and falling limb of a flood hydrograph. [3 marks]
Answer:
The rising limb is the increase in discharge as runoff reaches the channel (1); peak discharge is the highest discharge during the event (1); the falling limb is the decrease in discharge as the flow declines towards baseflow (1).
Question 25 · Spec 3.1.1.2 · Flood hydrograph
Define lag time. [1 mark]
Answer:
The time between peak rainfall and peak discharge.
Question 26 · Spec 3.1.1.2 · Flood hydrograph
Distinguish between baseflow and stormflow. [2 marks]
Answer:
Baseflow is the normal, steady flow supplied by groundwater (1); stormflow is the extra flow from rainfall that reaches the river quickly by overland flow and throughflow (1).
Question 27 · Spec 3.1.1.2 · Flood hydrograph
Define bankfull discharge. [1 mark]
Answer:
The discharge at which a river just fills its channel, above which it will flood.
Question 28 · Spec 3.1.1.2 · Flood hydrograph
Compare a flashy hydrograph with a subdued hydrograph. [2 marks]
Answer:
A flashy hydrograph has a short lag time, a steep rising limb and a high peak discharge (1); a subdued hydrograph has a long lag time, a gentle rising limb and a lower peak (1).
Question 29 · Spec 3.1.1.2 · Flood hydrograph
Explain why a steep basin with impermeable rock produces a flashy hydrograph. [3 marks]
Answer:
Little infiltration on impermeable rock means more overland flow (1); steep slopes make water travel quickly to the channel (1); so a large volume reaches the river in a short time, giving a short lag time and high peak (1).
Question 30 · Spec 3.1.1.2 · Runoff variation
Explain how rainfall intensity affects the shape of a flood hydrograph. [2 marks]
Answer:
Heavy, intense rain can exceed the infiltration capacity, creating more overland flow (1); so more water reaches the river quickly, giving a steeper rising limb and higher peak (1).
Question 31 · Spec 3.1.1.2 · Runoff variation
Explain how antecedent moisture conditions affect runoff. [2 marks]
Answer:
If soils are already saturated from earlier rain, infiltration is low (1); so more rainfall becomes overland flow and the peak discharge is higher with a shorter lag time (1).
Question 32 · Spec 3.1.1.2 · Runoff variation
Explain how drainage basin shape affects the hydrograph. [2 marks]
Answer:
In a circular basin, water from all parts arrives at the channel at about the same time, giving a high peak and short lag time (1); in an elongated basin, water arrives at different times, giving a lower, flatter peak and longer lag time (1).
Question 33 · Spec 3.1.1.2 · Runoff variation
Explain how drainage density affects the speed of runoff. [2 marks]
Answer:
A high drainage density means many streams, so water has a short distance to reach a channel (1); this speeds up runoff and gives a shorter lag time (1).
Question 34 · Spec 3.1.1.2 · Runoff variation
Explain how rock type affects the flood hydrograph. [2 marks]
Answer:
Permeable rocks such as chalk allow percolation and groundwater storage, delaying and reducing peaks (1); impermeable rocks such as clay or granite cause more surface runoff and a flashier response (1).
Question 35 · Spec 3.1.1.2 · Runoff variation
Explain how vegetation cover affects the flood hydrograph. [2 marks]
Answer:
Dense vegetation increases interception and evapotranspiration, reducing the volume of runoff (1); roots increase infiltration, and the slower flows lengthen lag time and reduce peaks (1).
Question 36 · Spec 3.1.1.2 · Runoff variation
Explain how snowmelt or frozen ground can cause rapid runoff. [2 marks]
Answer:
Frozen ground is impermeable, so meltwater and rain cannot infiltrate (1); rapid melting adds a large volume of water to overland flow, causing high river discharge (1).
Question 37 · Spec 3.1.1.2 · Changes over time
Explain how storm events cause short-term variations in river discharge. [2 marks]
Answer:
Intense rainfall adds a large input in a short time (1); runoff reaches the channel quickly, so discharge rises rapidly to a peak and then falls back (1).
Question 38 · Spec 3.1.1.2 · Changes over time
Explain how seasonal change affects the water cycle in a temperate drainage basin. [3 marks]
Answer:
Higher temperatures and plant growth in summer increase evapotranspiration, so soil water is lower and river flow falls (1); in winter, evapotranspiration is low and soils are saturated (1); so runoff, groundwater levels and river discharge are higher (1).
Question 39 · Spec 3.1.1.2 · Human impact
Explain how farming practices can alter the water cycle in a drainage basin. [3 marks]
Answer:
Ploughing downslope and soil compaction by livestock reduce infiltration and increase overland flow (1); field drains speed water to rivers, increasing peaks (1); crop cover and hedgerows increase interception and slow runoff (1).
Question 40 · Spec 3.1.1.2 · Human impact
Explain how deforestation affects the water cycle. [3 marks]
Answer:
Less interception and transpiration (1); less infiltration, as soils are exposed and compacted, so more overland flow (1); increased runoff and soil erosion, with a flashier hydrograph and greater flood risk (1).
Question 41 · Spec 3.1.1.2 · Human impact
Explain how urbanisation affects the flood hydrograph. [3 marks]
Answer:
Impermeable surfaces such as tarmac and concrete prevent infiltration (1); drains and sewers carry water rapidly to rivers (1); giving a short lag time, steeper rising limb and higher peak discharge (1).
Question 42 · Spec 3.1.1.2 · Human impact
Explain how afforestation affects the water cycle. [2 marks]
Answer:
Trees increase interception, transpiration and infiltration, reducing runoff (1); the hydrograph becomes more subdued, with a longer lag time and lower peak (1).
Question 43 · Spec 3.1.1.2 · Human impact
Explain how water abstraction affects stores and flows in a drainage basin. [3 marks]
Answer:
Pumping groundwater faster than it is recharged lowers the water table (1); this reduces baseflow so rivers carry less water or dry up (1); abstraction from rivers and lakes also lowers channel and surface storage (1).
Question 44 · Spec 3.1.1.2 · Quantitative skills
A river has a cross-sectional area of 12 m² and a mean velocity of 1.5 m per second. Calculate its discharge. [2 marks]
Answer:
Discharge = cross-sectional area × velocity (1); = 12 × 1.5 = 18 cumecs (m³ per second) (1).
Question 45 · Spec 3.1.1.2 · Quantitative skills
Convert a river discharge of 2,400 litres per second into cubic metres per second (cumecs). [1 mark]
Answer:
2.4 cumecs (1,000 litres = 1 m³).
Question 46 · Spec 3.1.1.2 · Field data
Describe how river discharge can be measured in the field. [3 marks]
Answer:
Measure the channel width and depths across it to find the cross-sectional area (1); measure velocity with a flow meter or float over a set distance (1); multiply area by velocity to find discharge in cumecs (1).
Question 47 · Spec 3.1.1.2 · Field data
Describe how infiltration rate can be measured in the field. [3 marks]
Answer:
Push an infiltration ring into the soil surface (1); fill it with a measured depth of water and record the time it takes to drain away (1); calculate infiltration in mm per hour, repeating at several sites for reliability (1).
Question 48 · Spec 3.1.1.2 · Field data
Suggest two reasons why field measurements of river discharge may be unreliable. [2 marks]
Answer:
Any two from: velocity varies across and within the channel, so one reading may not be representative (1); the channel is irregular, so depths are estimated (1); readings taken at one time may not show variation after rainfall (1); human error in timing or measuring (1).
Question 49 · Spec 3.1.1.2 · Case study themes
Explain how the impact of precipitation on drainage basin stores can affect sustainable water supply in a river catchment. [3 marks]
Answer:
Winter rain recharges soil water and groundwater, which are stores used for supply (1); reservoirs and aquifers can be depleted if droughts or abstraction exceed recharge (1); catchment management, such as limiting abstraction and increasing storage, helps sustain supply (1).
Question 50 · Spec 3.1.1.2 · Case study themes
Explain how catchment management can reduce flood risk. [3 marks]
Answer:
Any three from: planting trees or restoring wetlands to increase interception and storage (1); leaky dams and floodplain storage to slow flows (1); sustainable drainage in urban areas to reduce runoff (1); river channel restoration or flood storage areas to reduce peaks downstream (1).

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Drainage basins, water balance and hydrographs

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